K2-138 g: Spitzer Spots a Sixth Planet for the Citizen Science System

TL;DR

The paper reports the discovery and validation of a sixth planet in the K2-138 system using Spitzer photometry, highlighting its significance for planetary system studies and future mass measurement efforts.

Contribution

This work confirms the sixth planet in K2-138 and demonstrates its potential as a benchmark for planetary dynamics and atmospheric studies.

Findings

Six planets confirmed in K2-138 system

Orbital period of sixth planet is 41.966 days

System is ideal for mass and atmospheric characterization

Abstract

$K2$ greatly extended $Kepler$'s ability to find new planets, but it was typically limited to identifying transiting planets with orbital periods below 40 days. While analyzing $K2$ data through the Exoplanet Explorers project, citizen scientists helped discover one super-Earth and four sub-Neptune sized planets in the relatively bright ($V=12.21$, $K=10.3$) K2-138 system, all which orbit near 3:2 mean motion resonances. The $K2$ light curve showed two additional transit events consistent with a sixth planet. Using $Spitzer$ photometry, we validate the sixth planet's orbital period of $41.966\pm0.006$ days and measure a radius of $3.44^{+0.32}_{-0.31}\,R_{\oplus}$, solidifying K2-138 as the $K2$ system with the most currently known planets. There is a sizeable gap between the outer two planets, since the fifth planet in the system, K2-138 f, orbits at 12.76 days. We explore the possibility of additional non-transiting planets in the gap between f and g. Due to the relative brightness of the K2-138 host star, and the near resonance of the inner planets, K2-138 could be a key benchmark system for both radial velocity and transit timing variation mass measurements, and indeed radial velocity masses for the inner four planets have already been obtained. With its five sub-Neptunes and one super-Earth, the K2-138 system provides a unique test bed for comparative atmospheric studies of warm to temperate planets of similar size, dynamical studies of near resonant planets, and models of planet formation and migration.